1. Field of the Invention
The present invention relates to a process for manufacturing an optical article free of optical defects, in particular an ophthalmic lens, and more precisely for coating a geometrically defined surface forming a Fresnel lens lying on the substrate of said optical article.
The present invention also relates to a process for transferring a stack of coatings onto a geometrically defined surface forming a Fresnel lens lying on the substrate of an optical article.
2. Description of Related Art
It is a common practice in the art to coat at least one main surface of a lens substrate, such as an ophthalmic lens or lens blank, with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. These coatings are in general designated as functional coatings.
Thus, it is usual practice to coat at least one main surface of a lens substrate, typically made of an organic glass material, with successively, starting from the surface of the lens substrate, an impact-resistant coating (impact resistant primer), an abrasion- and/or scratch-resistant coating (hard coat), an anti-reflection coating and, optionally, an anti-fouling top coat. Other coatings, such as a polarized, photochromic or dyeing coating may also be applied onto one or both surfaces of the lens substrate.
Numerous processes and methods have been proposed for coating smooth or not very rough surfaces using coating liquids or liquid adhesive compositions for transfer coating.
U.S. Pat. No. 6,562,466 describes a process for transferring coatings from at least one support or mold part onto at least a geometrically defined surface of a lens blank having a surface roughness Sq lower than or equal to 1 μm, comprising:    providing a support or mold part having an internal surface bearing a coating and an external surface;    depositing onto said geometrically defined surface of the lens blank or onto said coating a pre-measured amount of a curable adhesive composition;    moving relatively to each other the lens blank and the support to either bring the coating into contact with curable adhesive composition or bring the curable adhesive composition into contact with the geometrically defined surface of the lens blank;    applying a sufficient pressure onto the external surface of the support so that the thickness of a final adhesive layer once the curable composition cured is less than 100 micrometers;    curing the layer of adhesive composition; and    withdrawing the support or mold part to recover the lens blank with the coating adhered onto the geometrically defined surface of said lens blank.
US 2005/140033 describes a process for coating the fined but unpolished geometrically defined main face of an optical article, having a surface roughness Rq lower than or equal to 1.5 μm, comprising:    depositing on the main face of the optical article or on the internal surface of the mold part an amount of a liquid curable coating composition;    moving relatively to each other the optical article and the mold part to either bring the coating composition into contact with the main face of the optical article or into contact with the internal face of the mold part;    applying pressure to the mold part to spread the liquid curable coating composition on said main face and form a uniform liquid coating composition layer onto the main face;    curing the liquid coating composition layer; and    withdrawing the mold part to recover a coated optical article free of visible fining lines.
U.S. Pat. No. 5,147,585, WO 97/33742 and JP 8090665 disclose an overmolding process to make composite lenses such as progressive or bi-focal lenses. All the surfaces to be coated or overmolded are smooth curved surfaces.
Resin cure shrinkage is a phenomenon that inevitably occurs upon polymerization and curing of a curable composition. This phenomenon is due to density change and affects the coating resin in a three-dimensional way. The extent of the shrinkage depends on the nature of the coating resin material. For example, CR-39®'s shrinkage rate is about 12% in volume and other methacrylate monomers undergo a shrinkage rate of 7 to 14% depending on the formulations and chemical structures.
When the surface to be coated is smooth or little rough (surface roughness typically <2.0 μm), this shrinkage phenomenon is not a big concern. However, when the surface to be coated has a roughness level higher than 2 microns, e.g. surfaces forming a Fresnel lens that typically have roughness levels ranging from 5 to 300 μm, coating shrinkage is an important issue. The higher the size of the Fresnel structure is, the bigger the shrinkage issue will be. Especially concerned are Fresnel structures having a surface roughness level of at least 50 μm.
Although not wishing to be bound by any theory, it is believed that moving the carrier and the optical article which has the Fresnel structure on its surface relatively to each other and then applying pressure creates many small liquid cells filled with liquid curable composition. Upon polymerization and curing, the liquid cells shrink. When some cells are fully blocked or sealed by the carrier due to the external force, the shrinkage causes the formation of air bubbles, arranged into a ring. These cosmetic defects are unacceptable for optical applications.
On the other side, it is not possible to obtain a smooth and even coating surface when coating a Fresnel lens forming surface using traditional spin or dip coating processes. Due to the particular geometry of said structured surface, optical defects, such as optical distortion, result from unevenness of the coating.
Besides, a few processes and methods have been proposed for coating diffractive surfaces on optical articles.
EP 1830205 discloses the preparation of a multilayer diffractive optical element by overmolding. A curable composition is applied between a mold part and a glass base material bearing a cured coating having a surface in a grating shape. Then, the composition is cured and the mold part is released, resulting in an optical article, in which two optical members sandwich a layer constituting a diffraction grating having a grating height of 20 μm.
JP 2007-212547 discloses the coating or lamination of flat diffractive surfaces of lenses, where the liquid composition flow is easily controlled. A layer of curable composition is charged between two lens members, one of which having an internal diffractive surface bearing a diffraction grating. Upon curing, one of the two lens members is moved to compensate for shrinkage of the coating. In the final optical article, the two lens members are stuck to each other through the cured coating composition.
However, no pressure is applied on the lens/optical members or the mold part during curing and casting of the curable composition in EP 1830205 and JP 2007-212547. As a consequence, coating shrinkage does not cause formation of bubbles. However, it will be very hard to control the coating resin thickness precisely in the entire lens surface, especially when the structure is on a curved surface, which is required to make a high optical quality lens.